1. The Field of the Invention
The present invention is related to solid rocket motor propellants and methods for formulating such propellants. More particularly, the present invention relates to solid rocket motor propellants which contain high levels of solids and which incorporate diepoxy curing agents.
2. Technical Background
Solid propellants are used extensively in the aerospace industry. Solid propellants have developed as the preferred method of powering most missiles and rockets for military, commercial, and space applications. Solid rocket motor propellants have become widely accepted because of the fact that they are relatively simple to formulate and use, and they have excellent performance characteristics. Furthermore, solid propellant rocket motors are generally very simple when compared to liquid fuel rocket motors. For all of these reasons, it is found that solid rocket propellants are often preferred over other alternatives, such as liquid propellant rocket motors.
Typical solid rocket motor propellants are generally formulated having an oxidizing agent, a fuel, and a binder. At times, the binder and the fuel may be the same. In addition to the basic components set forth above, it is conventional to add various plasticizers, curing agents, cure catalysts, ballistic catalysts, and other similar materials which aid in the processing and curing of the propellant. A significant body of technology has developed related solely to the processing and curing of solid propellants, and this technology is well known to those skilled in the art.
One type of propellant that is widely used in the solid rocket motor technology incorporates ammonium perchlorate (AP) as the oxidizer. The ammonium perchlorate oxidizer may then, for example, be incorporated into a propellant which is bound together by a polymer binder. Such binders are widely used and commercially available. It has been found that such propellant compositions provide ease of manufacture, relative ease of handling, good performance characteristics and are at the same time economical and reliable. In essence it can be said that ammonium perchlorate composite propellants have been the backbone of the solid propulsion industry for approximately the past 40 years.
In some instances it is necessary to provide high levels of thrust in order to propel the desired payload into space. Such applications include, for example, propellants for use in the space shuttle program. Obviously, the space shuttle is an enormous and complex device that requires extraordinary thrust in order to propel it into earth orbit. At the same time, the propellant used in the space shuttle is not under the same constraints imposed by combat or other specialized propellant uses. In that regard, smoke production is not a major concern. The primary concern is obtaining the maximum thrust per unit of propellant used while maintaining acceptable safety characteristics.
It has been found that an acceptable propellant for such uses is a relatively high solids propellant having a butadiene-acrylonitrile-acrylic acid terpolymer (PBAN) binder. The solids incorporated into the propellant are typically ammonium perchlorate salt as an oxidizer, and aluminum as a metallic fuel. When the propellant is burned, these solid ingredients are the primary contributors to the thrust produced. Propellants of this type are found to produce a high level of thrust per pound of propellant and are preferred for high thrust applications.
A typical PBAN shuttle propellant includes about 84% to 86% by weight solids. In one typical example, the propellant may include about 16% aluminum, 69.75% ammonium perchlorate, and 0.25% iron oxide, for a total solids loading of 86%. The solids are then bound together by the polymeric PBAN binder and a corresponding Bisphenol A-diglycidyl ether curative, which together comprise the remaining 14% of the propellant formulation.
In view of the fact that the solids provide most of the energy output of the propellant, it is desirable to maximize the percentage of solids in the propellant formulation. If it is possible to increase the solids loading by even a few percent, it is possible to obtain marked improvements in energy output. The result of such improvements in performance is that the amount of propellant per pound of payload can be reduced. Thus, a larger payload can be propelled into space, or existing payloads can be propelled more efficiently.
One of the primary problems with PBAN propellants relates to processiblity. High solids PBAN propellants tend to be very viscous. The viscosity of such propellants can rapidly reach unacceptable levels if there is an increase in the level of solids loading. For example, observed end of mix viscosities of PBAN propellants having 86% solids are in the range of 17 to 30 kilopoise (Kp), which are in themselves relatively high and present problems in processing. However, when the solids level is raised even two percent to 88% by weight, the viscosity rises to over 50 Kp. Viscosities in this range are unacceptable because the mixture is not adequately processible.
As mentioned above, the conventional curatives in propellants of this type are based on bisphenol A. It is observed that the combination of PBAN and bisphenol A-based curatives contributes significantly to the high viscosity observed during processing. The high viscosity of the binder-curative combination clearly limits the ability to add solids to the composition.
The high viscosities observed when the solids level reaches or exceeds about 86% makes such propellant formulations unworkable and unacceptable. Propellants having high viscosities will not readily flow, making it difficult to load rocket motors with these materials. In addition, it is often observed that viscous propellants entrap air during processing. Air voids can be dangerous when the propellant is burned, providing hot spots and uneven burning of the propellant. Thus, until now it was not possible to consistently prepare high solids PBAN propellants having solids levels over about 86% by weight.
Accordingly, it would be a significant advancement in the art to provide PBAN propellants having solid levels at or above 86%. In that regard, it would be a significant advancement in the art to provide such propellants which were also processible such that problems of very high viscosity were minimized. It would be a further advancement in the art to provide PBAN-curative combinations which resulted in reduced viscosity and which enabled the propellant to contain additional solids.
Such methods and compositions are disclosed and claimed herein.